The present invention is directed to films which contain single site catalyzed copolymers. The present invention is especially directed to such films suitable for use in the packaging of products, especially fresh meat products and processed meat products. The present invention is also directed to packages comprising such films, especially heat shrinkable films.
Polyethylene-based films have found many useful applications in the packaging of meats, cheeses, and poultry, as well as other food products and non-food products. Polyethylene heat-shrinkable films have been used to obtain a tight package by placing the product in, for example, a bag made from the heat-shrinkable film, followed by sealing the bag and thereafter passing the product, in the sealed bag, through a shrink tunnel in which the bag shrinks to form a tight package around the product. Heat-shrinkable films comprising heterogeneous ethylene/alpha-olefin copolymer, such as linear low density polyethylene (LLDPE), are known to be particularly useful for making heat-shrinkable bags for packaging large cuts of fresh red meat, including bone-in meat products.
Broadly considered, for more than 10 years homogeneous ethylene/alpha-olefin copolymers have been known to those of skill in the polyethylene art. Homogeneous ethylene/alpha-olefin copolymers contain multiple polymer chains with each polymer chain having virtually the same architecture, e.g., virtually identical molecular weight and virtually identical comonomer mer distribution. Therefore, the copolymer chains produced from single site systems are uniform not only in chain length, but also in average comonomer content, and even regularity of comonomer spacing, i.e., incorporation of comonomer mer along the chain.
However, the early homogeneous polymers are the result of polymerization processes so inefficient the resulting polymers have generally had a density too low, and been too expensive, to be commercially competitive in the vast majority of commercial polyethylene packaging films. More recently, i.e., in the mid to late 1980""s, novel single site catalysts were used in the production of homogeneous ethylene/alpha-olefin copolymers, these catalysts being referred to as metallocene catalysts, single site catalysts, constrained geometry catalysts, etc. Using such catalysts, the homogeneous ethylene/alpha-olefin copolymers can be produced so efficiently that the cost of the resulting homogeneous polymers is competitive with heterogeneous polyethylene copolymers which have been used for many years in the manufacture of a wide variety of packaging films.
Generally speaking, metallocene catalysts, etc. are organometallic compounds containing one or more cyclopentadienyl ligands attached to metals such as hafnium, titanium, vanadium, or zirconium. A co-catalyst, such as but not limited to, oligomeric methyl alumoxane is often used to promote the catalytic activity. By varying the metal component and the cyclopentadienyl ligand a diversity of polymer products may be tailored having molecular weights ranging from about 200 to greater than 1,000,000 and molecular weight distributions, i.e, Mw/Mn, of from about 1.5 to 15. The choice of co-catalyst influences the efficiency and thus the production rate, yield, and cost.
Examples of metallocene catalysts include, for example, bis(cyclopentadienyl) dichloro-transition metal, bis(cyclopentadienyl) methyl, chloro-transition metal, and bis(cyclopentadienyl) dimethyl-transition metal, where the metals include choices such as titanium, zirconium, hafnium, and vanadium. Examples of non-metallocene catalysts include TiCl4, TiBr4, Ti(0C4H9)2Cl2, VCl4, and VOCl3.
Moreover, metallocene catalysts, etc., have enabled the production of homogeneous ethylene/alpha-olefin copolymers over a wide range of density. That is, most of the early homogeneous ethylene/alpha-olefin copolymers have a relatively low density. However, the novel metallocene catalysts can be used to polymerize homogeneous ethylene/alpha-olefin copolymers over a much wider density range. This wider density range includes the densities of various heterogeneous ethylene/alpha-olefin copolymers previously used in commercialized packaging films, i.e, heterogeneous ethylene/alpha-olefin copolymers such as LLDPE and very low density polyethylene (VLDPE). In other words, although the early homogeneous ethylene/alpha-olefin copolymers generally have a density under 0.90 g/cc, the new metallocene catalysts, etc. can be used to provide homogeneous ethylene/alpha-olefin copolymers having densities at and above 0.90 g/cc., at a cost competitive with LLDPE and VLDPE. Accordingly, commercial interest in homogeneous polymers has increased due to the relatively low cost of the homogeneous polymers available through the use of metallocene catalysts, etc. This increased commercial interest is evident in the area of packaging films, as homogeneous polymers are now cost-effective, and are known as having a much more narrow molecular weight distribution, and a much more regular comonomer distribution, than the heterogeneous polymers which have to-date been the principal ethylene/alpha-olefin copolymers used by the plastics industry. As such, homogeneous polymers present the potential of providing properties more highly xe2x80x9ctailoredxe2x80x9d to specific end uses.
It remains desirable to provide packaging films having improved characteristics, i.e, to provide films having improved physical properties, such as increased impact strength, greater clarity and gloss, increased free shrink and shrink at a lower temperature, improved printability, improved extrudability and processability, etc.
It has been discovered that homogeneous ethylene/alpha-elfin copolymers can be used to provide a heat-shrinkable film having a surprisingly high impact strength while also having a high total free shrink at 185xc2x0 F. The surprisingly high impact strength is obtained by preparing a heat-shrinkable film comprising a homogeneous ethylene/alpha-olefin copolymer. Such heat-shrinkable films can be prepared, for example, by extruding an unoriented xe2x80x9ctapexe2x80x9d comprising the homogeneous ethylene/alpha-olefin copolymer, and thereafter orienting the tape at a relatively low temperature, in order to provide the film with the a relatively high free shrink. By orienting the tape at a relatively low temperature, an impact strength is achieved which is substantially higher than the impact strength obtainable upon orienting a similar film comprising a heterogeneous ethylene/alpha-olefin copolymer of comparable density prepared in a comparable process. Applicants have discovered that heat shrinkable monolayer films according to the present invention, i.e, comprising homogeneous ethylene/alpha-olefin copolymer, have surprisingly high impact strength, e.g., impact strengths of from 3 to 13 times the impact strength of comparably-produced monolayer heat-shrinkable films comprising heterogeneous ethylene/alpha-olefin copolymer of comparable density and comparable melt index.
The higher impact strength achieved by the film of the present invention can be used to provide a film having an increased impact strength for any given thickness. In turn, this advantage renders it possible to maintain an impact strength while downgauging the film thickness, thereby conserving resin while maintaining the impact strength performance.
As a first aspect, the present invention is directed to a heat-shrinkable film comprising a homogeneous ethylene/alpha-olefin copolymer, wherein the film has an impact strength of from about 35 to 200 pounds and a total free shrink, at 185xc2x0 F., of from about 80 to 150 percent. Preferably, the heat-shrinkable film has an impact strength of from about 40 to 90 pounds, and preferably, has a total thickness of from about 1 to 3 mils. More preferably, the film has a total free shrink, at 185xc2x0 F., of from about 80 to 120 percent, and preferably has an impact strength of from about 50 to 110 pounds. Still more preferably, the heat-shrinkable film has an impact strength of from about 60 to 110 pounds.
The heat-shrinkable film can be a monolayer film or a multilayer film. If a multilayer film, the film layer comprising the homogeneous ethylene/alpha olefin copolymer can be an inner layer or an outer layer. If in an inner layer, the homogeneous ethylene/alpha olefin copolymer can be present in the inner layer in an amount of from about 60 to 100 weight percent, based on the weight of the inner layer, with the inner layer having a thickness of from about 1 to 3 mils; more preferably, the homogeneous ethylene/alpha olefin copolymer is present in the inner layer in an amount of about 80 to 100 weight percent, based on the weight of the inner layer, with the inner layer having a thickness of about 1 to 2.5 mils; still more preferably, the homogeneous ethylene/alpha olefin copolymer is present in the inner layer in an amount of about 90 to 100 weight percent, based on the weight of the inner layer; yet still more preferably, the homogeneous ethylene/alpha olefin copolymer is present in the inner layer in an amount of about 100 weight percent, based on the weight of the inner layer, with the inner layer having a thickness of from about 1 to 2.5 mils.
The homogeneous ethylene/alpha-olefin copolymer can be present in the film layer alone or as a first component present in a composition which further comprises a second component. Preferably, the second component comprises at least one member selected from the group consisting of another ethylene/alpha-olefin copolymer, ethylene/vinyl acetate copolymer, ethylene/vinyl ester copolymer, ethylene/acrylic acid copolymer, propylene/ethylene copolymer, polyethylene homopolymer, polypropylene homopolymer, and ionomer, with the second component being present in the composition in an amount of from about 1 to 40 weight percent, based on the weight of the composition. Preferably, the first component and the second component are in a substantially uniform blend. The second component can also be any one of the above polymers which is anhydride grafted, more preferably maleic anhydride grafted. Methacrylic acid copolymer is a preferred acrylic acid copolymer for the second component. Methyl acrylate and butyl acrylate are preferred vinyl esters for the second component.
A preferred multilayer heat-shrinkable film further comprises an O2-barrier layer and an outer sealant layer. More preferably, the outer sealant layer is a first outer layer, and an outer abuse layer is a second outer layer.
Another more preferred multilayer film further comprises an O2-barrier layer and an outer sealant layer, and at least one tie layer between the barrier layer and the first outer layer. Still more preferably, the tie layer between the barrier layer and the first outer layer is a first tie layer, and a second tie layer is between the barrier layer and the second outer layer.
A preferred film, having the homogeneous ethylene/alpha-olefin copolymer in an outer film layer, comprises a first outer film layer comprising a first homogeneous ethylene/alpha-olefin copolymer, and a second homogeneous ethylene/alpha-olefin in a second outer film layer. More preferably, the first homogeneous ethylene/alpha-olefin copolymer is substantially identical to the second homogeneous ethylene/alpha-olefin copolymer. Still more preferably, the first and second outer layers are substantially identical in chemical composition and thickness, and the heat-shrinkable film is substantially symmetrical.
A particularly preferred multilayer film according to the present invention comprises a first inner layer, a second inner layer, and a third inner layer. The first inner layer is a first tie layer, the second inner layer is an O2-barrier layer, and the third inner layer is a second tie layer. The first and second outer layers are substantially identical in chemical composition and thickness, and the first and second tie layers are substantially identical in chemical composition and thickness, and the heat-shrinkable film is substantially symmetrical.
Preferably, the heat-shrinkable film has a total free shrink, at 185xc2x0 F., of at least 50 percent; more preferably, from about 55 to 200 percent; still more preferably, from about 55 to 110 percent.
Preferably, the heat-shrinkable film comprises at least one layer comprising a crosslinked polymer network. Preferably, the crosslinking is formed by irradiating the polymer.
The homogeneous ethylene/alpha-olefin copolymer preferably has a density of from about 0.86 to 0.93 g/cc; more preferably, from about 0.88 to 0.91 g/cc; and still more preferably, from about 0.89 to 0.91 g/cc.
The alpha-olefin of the homogeneous ethylene/alpha-olefin preferably comprises at least one member selected from the group consisting of C3-C3 alpha-olefins; more preferably, C4-C12 alpha-olefins; and still more preferably C4-C8 alpha-olefins.
As a second aspect, the present invention is directed to a heat-shrinkable film comprising homogeneous ethylene/alpha-olefin copolymer, the heat-shrinkable film having an impact strength of from about 80 to 250 pounds and a total free shrink, at 185xc2x0 F., of from about 30 to 150 percent. Preferably, the heat-shrinkable film has an impact strength of from about 80 to 200 pounds, and a total free shrink, at 185xc2x0 F., of from about 50 to 120 percent.
This heat-shrinkable film can be a monolayer film or a multilayer film. Preferably, it is a multilayer film, and the layer comprising the homogeneous ethylene/alpha olefin copolymer is an inner layer.
As a third aspect, the present invention is directed to an article. The article comprises a heat-shrinkable film comprising a homogeneous ethylene/alpha-olefin copolymer, wherein the heat-shrinkable film has an impact strength of from about 35 to 200 pounds and a total free shrink, at 185xc2x0 F., of from about 80 to 150 percent, and wherein the article is a member selected from the group consisting of a bag and a casing.